The subject of electromagnetic compliance (EMC) has in recent years become very significant, especially in the field of electronics. Many environments now include large numbers of electronic devices, especially personal desktop computers and similar apparatuses. It is known that the power supplies that form a part of such devices switch at exceedingly high frequencies. Such switching causes EMI. Stray EMI can interfere with the correct operation of neighboring apparatuses. The effects of such interference can range from mere inconvenience to users to catastrophic or even life-threatening consequences.
As a result, many governments have passed legislation requiring manufacturers of electronic equipment to limit the amplitude of EMI created by its products and/or to filter the EMI so that it is attenuated in frequency ranges that would otherwise hamper or affect the operation of other devices. In view of these requirements, it has become commonplace to include EMI filters connected in printed circuit boards of power supplies of devices such as desktop computers. It is generally more beneficial, however, to avoid generating EMC than to filter it by additional circuitry. Typically, power supplies use a semiconductor device to switch current through an inductor. At the moment of switch off, the current flowing in the inductor is interrupted, and the voltage across the inductor changes rapidly. The limit on the rate of change of voltage is usually imposed by parasitic capacitances, typically between the wire forming one turn and that forming an adjacent turn, which turns form a resonant circuit with the inductor or part of the inductor. The net effect is to cause energy to be emitted from the circuit at one or more pseudo-resonant frequencies. This spurious energy is in addition to the wanted energy that is transferred to the load. Often the spurious energy is in a frequency band which is controlled by legislative limits.
It is known in the art to replace the regularly spaced windings of a conventional EMI filter toroid with “piled” windings, i.e. windings that overlie one another in a substantially irregular manner, over a major part of the toroid. However, this solution leads to a very large number of small resonant circuits. Thus, the undesirable self-resonances are reduced in energy, but increased in number. It is therefore necessary to apply further filtering or other suppression measures in order to reduce this energy to acceptable levels.
Such windings are commercially available, for example, for power factor correction circuits. An increasing proportion of electronic devices is equipped with power factor correction circuitry. Older devices typically use a rectifier and capacitor combination as an alternating current (AC) to direct current (DC) converter to provide a DC supply for the AC to DC converter that actually powers the device. Despite their simplicity, such AC to DC converters draw large peak currents from the AC supply when the AC voltage is at or near its peak, and little current elsewhere in the cycle. The resulting distortion of the current waveform from an ideal sinusoidal shape causes higher root mean square (RMS) currents in the supply wiring than would be expected from the electrical power drawn by an electronic device.
This effect may not be significant when considering a single device such as a personal computer. On the other hand, it is now commonplace for entire buildings, on completion, to be equipped with large numbers of identical apparatuses, such as a bulk order of identical personal computers. The power factor effects of the plurality of AC to DC converters that such an installation represents are cumulative. Consequently the opening of ego a new call or data centre may for example cause significant supply current distortion, purely as a result of a large number of AC to DC converters being connected to an alternating mains supply.
Electricity companies have for many years sought to eliminate the inefficiency of transmission that this represents. In the case of personal computers, however, it is not readily possible to use the kinds of power factor correction apparatus, such as capacitive shunts, that are suitable for ego electric motors. It follows therefore that there is a need for an improved means of reducing the supply current distortion. Typically this need is met by a switched mode power factor correction circuit that makes the shape of the current waveform substantially the same as, and in phase with, the voltage waveform. As well as its beneficial effects, the power factor correction circuit often gives rise to significant EMI.